*Principle of Original Horizontality Scientists also know that sedimentary rock generally forms in horizontal layers. The Principle of Original Horizontality is that sedimentary rocks left undisturbed will remain in horizontal layers. Therefore, scientists can assume that sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed.
Unconformities Movements of Earth’s crust can lift up rock layers that were buried and expose them to erosion. Then, if sediments are deposited, new rock layers form in place of the eroded layers. The missing rock layers create a break in the geologic record in the same way that pages missing from a book create a break in a story. *A break in the geologic record is called an unconformity. An unconformity shows that deposition stopped for a period of time, and rock may have been removed by erosion before deposition resumed.